This invention relates to a single-electron controlling magnetoresistance element, and in particular to an element which functions based on a novel single-electron controlling method taking advantages of a charging energy effect or so-called coulomb blockade and of a negative magnetoresistance effect of a ferromagnetic tunnel junction.
Attempts to further increase the integration of elements based on the conventional Si-MOSFET are now confronted with a fundamental difficulty due to an increasing complication of structure resulting from an increasing miniaturization of device as well as due to difficulty in fine working for manufacturing such a miniature device. Under the circumstances, there is a growing need for a development of an element which is simple in structure and hence suited for use in miniaturizing and integrating a device. As one example of element to meet such a need, a single-electron tunnel element which functions based on an operational principle of so-called coulomb blockade has been proposed (K. K. Likharev, IBM J. Res. Develop., 32,144(1988)).
When the tunneling of a single electron (an electric charge "e") takes place through a single tunnel junction (a junction capacity: C; a junction tunnel resistance: R.sub.t), a change in electrostatic energy before and after the tunneling, which is represented by the following formula (1), becomes a problem. EQU E.sub.c =e.sup.2 /2C (1)
The change in electrostatic energy E.sub.c may become larger than a thermal energy in an ultra-fine tunnel junction where the junction capacity "C" thereof is very small. Depending on the magnitude of quantity of electrification at this junction, even the tunneling of a single electron is forbidden because of such a high energy to be generated by the tunneling of a single electron. This phenomenon is called coulomb blockade (K. K. Likharev, IBM J. Res. Develop., 32,144(1988)), wherein the tunneling of electron is expected to be taken place one by one like a particle, in spite of the fact that this tunneling phenomenon is essentially brought about by the wave motion of electron.
In order to render this coulomb blockade to be clearly revealed, the change in electrostatic energy E.sub.c is required to be sufficiently larger than any of thermal fluctuation and quantum mechanical fluctuation. These conditions can be expressed by the following formulas (2) and (3). EQU E.sub.c &gt;&gt;k.sub.B T (2) EQU E.sub.c &gt;&gt;h/R.sub.t C (3)
The condition represented by the formula (3) may be written also by the following formula (4). EQU R.sub.q &lt;&lt;R.sub.t ( 4)
wherein R.sub.q is quantum resistance which can be expressed by the following formula (5). EQU R.sub.q .tbd.h/(2e.sup.2)=12.9 k.OMEGA. (5) PA1 wherein h is Planck's constant. PA1 a couple of first ferromagnetic bodies each magnetized in a first direction; PA1 a second ferromagnetic body magnetized in a second direction in an initial direction and sandwiched between the couple of first ferromagnetic bodies with a tunnel junction interposed therebetween respectively; and PA1 means for directing the magnetization direction of the second ferromagnetic body to a direction different from the second direction; PA1 wherein a charging energy E.sub.c of a single electron in at least one of the tunnel junctions interposed between the first ferromagnetic body and the second ferromagnetic body meets the following conditions: EQU E.sub.c &gt;&gt;k.sub.B T (2) EQU E.sub.c &gt;&gt;h/R.sub.t C (3) PA1 a couple of first ferromagnetic bodies each magnetized in a first direction; PA1 a second ferromagnetic body magnetized in a second direction in an initial direction and sandwiched between the couple of first ferromagnetic bodies; PA1 a tunnel junction disposed on one side of the second ferromagnetic body which faces one of the first ferromagnetic bodies; and PA1 means for directing the magnetization direction of the second ferromagnetic body to a direction different from the second direction; PA1 wherein a charging energy E.sub.c of a single electron in the tunnel junction interposed between the first ferromagnetic body and the second ferromagnetic body meets the following conditions: EQU E.sub.c &gt;&gt;k.sub.B T (2) EQU E.sub.c &gt;&gt;h/R.sub.t C (3) PA1 a couple of first ferromagnetic bodies each magnetized in a first direction; PA1 a second ferromagnetic body magnetized in a second direction opposite to the first direction and sandwiched between the couple of first ferromagnetic bodies with a tunnel junction interposed therebetween respectively; and PA1 magnetization-inversion means disposed on the second ferromagnetic body for inverting the magnetization direction of the second ferromagnetic body so as to direct the magnetization direction of the second direction in the same direction as that of the first direction; PA1 wherein a charging energy E.sub.c of a single electron in at least one of the tunnel junctions interposed between the first ferromagnetic body and the second ferromagnetic body meets the following conditions, and PA1 a tunnel resistance at each tunnel junction is in the range of 13 k.OMEGA. to 150 k.OMEGA.: EQU E.sub.c &gt;&gt;k.sub.B T (2) EQU E.sub.c &gt;&gt;h/R.sub.t C (3)
In the case of the ordinary single-electron controlling element, it is essential that the quantum fluctuation is completely suppressed, and, in view of the aforementioned formula (4), the tunnel resistance R.sub.t is required to be set sufficiently higher than the quantum resistance R.sub.q.
The single-electron controlling element may be formed, as a basic structure, of; a double tunnel junction provided with a tunnel barrier layer which is constituted by a channel electron layer pinched off electrostatically from an electrode formed of a semiconductor or a normal conductive metal, or by an insulating body; and a gate electrode which is disposed facing to the central electrode (hereinafter referred to as an island) and capacitively coupled with the central electrode (SET transistor). Alternatively, the single-electron controlling element may be formed, as a basic structure, of a gate capacitor which is serially connected to the aforementioned double tunnel junction (SET memory). FIGS. 1A and 1B, and FIGS. 2A and 2B respectively represent examples of the structure and characteristics of these conventional single-electron controlling elements (K. K. Likharev, IEEE Trans. Magn., 23,1142(1987)). Specifically, FIG. 1A represents a circuit diagram of the C-SET, FIG. 1B represents characteristics of the C-SET, FIG. 2A represents a circuit diagram of the R-SET, and FIG. 2B represents characteristics of the R-SET. The control of single-electron is performed mainly by way of a gate, i.e. by changing an excessive electron generated at the island due to the tunneling of electron.
Since the single-electron controlling element is simple in structure, it is suited for use in miniaturizing and integrating a device. Additionally, the performance of the single-electron controlling element can be further improved with an increase in miniaturization. Moreover, since the control of the single-electron controlling element can be performed based on a single electron, a clear on/off operation can be achieved, so that the single-electron controlling element is now considered as one of promising elements in future.
However, there is a problem that although the operation of this single-electron controlling element is based on a mode where the coulomb blockade can be completely taken place all over the element, it is difficult as a matter of fact to completely render the coulomb blockade to take place all over the element of high integration. In other words, there has been no idea of positively utilize a non-linear mode wherein the coulomb blockade takes place incompletely. Further, the operational principle that has been conventionally considered of is based only on the electric charge of electron, and the utilization of effects that will be generated from the degree of freedom of spin has not been considered. Namely, a diversified control of the element is not fully taken into account up to date.
On the other hand, the ferromagnetic tunnel junction, which is one of the fundamental structural element of the conventional magnetic elements, is constructed as shown in FIG. 3 (S. Maekawa and U. Gafvert, IEEE Mag. MAG-18 707(1982)). Referring to FIG. 3, the reference numeral 71 represents a first ferromagnetic electrode formed of Co, and 72 represents a second ferromagnetic electrode formed of Ni, which is connected via a tunnel barrier 73 (formed of NiO) with the first ferromagnetic electrode 71. This conventional magnetic element has characteristics as shown by a diagram shown in FIG. 4. Since the state density in the ferromagnetic substance is dependent on the orientation of spin, the generation of a negative tunnel magnetic reluctance can be generally represented by the following formula (6). ##EQU1##
It is assumed that the generation of a negative tunnel magnetic reluctance is brought about by a change in state density due to the reversal of magnetization (inverse-parallel to parallel) by a magnetic field. This structure is more advantageous in the respect that the SN ratio thereof is larger than the multi-layered structure consisting of a magnetic layer/a non-magnetic layer/a magnetic layer, which is another example of structure exhibiting the same negative magnetic reluctance as mentioned above. However, since the rate of change of magnetic reluctance is only 15% or so in the ferromagnetic tunnel junction shown in FIG. 3, a structure which is capable of exhibiting a larger change in magnetic reluctance is now desired. Furthermore, since the aforementioned conventional structure is formed of a single tunnel junction structure to be driven with a low impedance and exhibits a phenomenon called "an environmental effect of electromagnetic field" against the coulomb blockade, a large fluctuation is caused to generate in electric charge. Due to these reasons, it is very difficult to take advantage of the coulomb blockade as the operational principle of the single-electron controlling element even if the area of tunnel junction is further minimized (S. Iwabuchi, H. Higurashi, Y. Nagaoka, JJAP Series 9, 126(1992); H. Higurashi, S. Iwabuchi and Y. Nagaoka, Phys., Rev. B51, 2387(1995)).
It would be impossible in the conventional magnetic element as shown in FIG. 3 to expect the coulomb blockade to be brought about by the effect of charging energy. However, there is a need to further promote the integration of semiconductor elements in concurrent with an effort to achieve an increased miniaturization of element which can be competitive with or replaceable for the existing silicon element in future. Therefore, it is not unreasonable to find a way to effectively utilize the coulomb blockade even in a magnetic element. Accordingly, if a basic element structure/operational mode which takes full advantage of the coulomb blockade can be realized, it is expected that not only the performance of unit element can be improved, but also the freedom in design of the whole system can be expanded.